I expect it's just a standard Seat post QR.
5mm shaft like the hub ones just shorter... super common.
threaded hole for the regular bolt is larger.., so it passes straight thru..
Skewer nut on the front.


Bought just the bag frame, you have discovered you can lash other bags to it..


I did so with an old messenger bag with a waist strap,
but that is a No Go with another messenger bag,
without such a belt, So, the waist belt is the thing..

with the frame you can strap* packaged on there , pop frame off and bring box in.. frame..and all ,



not too difficult to sew your own bag , on a home sewing machine.. too....

the velcro strap on the bottom keeps it on the frame.

*Surly Junk straps are useful ... they have toe strap buckles on Meter Plus long straps..





.....

Jahwind bars
 

That is a mighty clean handlebar setup which looks very sharp with that saddle. Flipped and chopped drop bars?

Carradice Barley it's a great option and modify a front carrier frame it's very easy.

Oh yea I have a picture now Bare Rack, steel, Modified locally ..

https://scontent-sea1-1.xx.fbcdn.net...6e&oe=5CDD6E03

Carradice Barley
 

Here some fotos, now i use the black version:
https://cimg2.ibsrv.net/gimg/bikefor...38e84ffeba.jpg
More infos: Folding Style ©: Carradice Barley Bag for Brompton - Modified Front Carrier Frame

Carradice has bags made to go on un altered brompton front support frames, too ..

M


https://www.carradice.co.uk/images/c...1733762416.jpg
S

https://www.carradice.co.uk/images/c..._687258671.jpg




....

I just wanted to thank everyone on this thread for my inspiration tonight.

Rolltop/messenger bag frame + free bag I used back in dental school= surprisingly versatile commuting bag on brompton.https://cimg3.ibsrv.net/gimg/bikefor...3196faf7c8.jpg

Has a sack of potting soil, no problem.

As long as you have a travel friendly bag (with slot for roller airline travel luggage handle), you can use that bag with any Brompton bag frame